The present invention relates to a novel copper-carbon fiber composite having a low linear thermal expansion coefficient and no directional property, and to a process for the preparation thereof.
Carbon fibers have a high strength, a high elastic modulus, a high heat resistance and a high wear resistance and are characterized by a desirable low thermal expansion coefficient. However, carbon fibers are insufficient in electric conductivity and thermal conductivity. On the other hand, although such metals as copper, silver and aluminum have a high thermal expansion coefficient, they are excellent in the electric conductivity. Namely, properties of these metals are quite opposite to properties of carbon fibers. Accordingly, if such metal is combined in certain proportions with carbon fibers, there will be obtained a material of a low thermal expansion, high electrical conductivity and high thermal conductivity. Especially, copper matrix-carbon fiber composites which are cheap and excellent in the elastic conductivity have a high practical value and it is expected that these materials can be applied to various equipments. For example, if such a composite is used as a substrate for a silicon semiconductor element, a semiconductor device having a high capacity will be prepared with ease at a low cost.
When the fiber direction is fixed in a copper-carbon fiber composite, the thermal expansion coefficient is low in the longitudinal direction of the fiber but is high in the direction perpendicular to said direction. In short, if carbon fibers have a unidirectional characteristic, the thermal expansion characteristic of the composite is anisotropic. Accordingly, when the composite is applied to a semiconductor device or the like, it is necessary to render fiber directions random to thereby expel the anisotropic characteristic of the thermal expansion coefficient. As one of means for attaining this object, there can be mentioned a method in which carbon fibers are arranged in a net-like form. This method, however, is defective in that production of composites is troublesome and difficult. When carbon fibers are randomly arranged in the copper matrix, the anisotropic characteristic of the thermal expansion coefficient is completely expelled. However, it has been found that if the composite is subjected to a temperature higher than the softening point of the copper matrix, an abnormal volume change takes place and this abnormal volume change results in deformation and breakdown of the composite. This undesired phenomenon takes place frequently especially when the volume ratio of carbon fibers is increased to reduce the thermal expansion coefficient of the composite. Further, in connection with the influences of temperatures, deformation and breakdown are caused by exposing the composite to a high temperature for a short time as in case of brazing, and further, even if the exposure temperature is relatively low, deformation and breakdown similarly take place when the composite is exposed for a long time.
As a result of our research efforts made on the above undesired phenomenon of deformation and breakdown by the temperature change, it was confirmed that breakdown is due to elastic deformation of carbon fibers. More specifically, in order to increase the amount of carbon fibers in the copper matrix, it is necessary to compress a mass of fibers entangled in random directions, and if such carbon fibers in which elastic deformation has been caused by this compression are sealed into the copper matrix, a large inner stress is left in the resulting composite and so far as the strength of the matrix is sufficiently stronger than the compression stress left in the fibers, deformation or breakdown is not caused in the composite but if there is a weak portion in the matrix in the composite, the stress is concentrated to this weak portion and finally overall breakdown takes place in the composite. Accordingly, it has been found that a risk of such deformation or breakdown is greatly reduced when the fiber amount is small or the ratio of the length to the diameter in the used fibers is low even if the fiber amount is relatively large.